General Chemistry Principles, Patterns, and Applications, 2011

products can vary over a wide range. In contrast, a system whose reactants, products, or both are in more
than one phase is a heterogeneous equilibrium, such as the reaction of a gas with a solid or liquid.
Because the molar concentrations of pure liquids and solids normally do not vary greatly with
temperature, their concentrations are treated as constants, which allows us to simplify equilibrium
constant expressions that involve pure solids or liquids. [2] (Recall from Chapter 11 "Liquids", for example,
that the density of water, and thus its volume, changes by only a few percentage points between 0°C and
100°C.)
Consider the following reaction, which is used in the final firing of some types of pottery to produce
brilliant metallic glazes:
Equation 15.20
CO2 g
( ) + C( s) 2CO( g)
The glaze is created when metal oxides are reduced to metals by the product, carbon monoxide. The
equilibrium constant expression for this reaction is as follows:
Equation 15.21
K = [ CO]2[CO2] C
[ ]
Because graphite is a solid, however, its molar concentration, determined from its density and molar
mass, is essentially constant and has the following value:
Equation 15.22
[ C] = 2.26 g / cm312.01 g / mol ´1000 cm3/ L = 188 mol / L =188 M
We can rearrange Equation 15.18 so that the constant terms are on one side:
Equation 15.23
K[C] = K(188) = [ CO]2[CO2]
Incorporating the constant value of [C] into the equilibrium equation for the reaction inEquation 15.17,
Equation 15.24
K ' = [ CO]2[CO2]
The equilibrium constant for this reaction can also be written in terms of the partial pressures of the
gases:
Equation 15.25
Kp = (PCO)2PCO2
Saylor URL: http://www.saylor.org/books
Saylor.org
1365